Chemisorption, photodesorption and conductivity measurements on ZnO surfaces

Shapira, Yoram; Cox, S. M.; Lichtman, David

doi:10.1016/0039-6028(76)90086-8

Cited by 112 publications

(36 citation statements)

References 17 publications

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“…We also note that it may be possible that oxygen containing compounds other than O 2 , e.g., H 2 O, could serve as oxygen source to replenish the lost oxygen [36], although CO 2 was clearly found ineffective [23]. Nonetheless, it is clear that only oxygen can take the place of the lost lattice oxygen and restore the resistivity, regardless of the actual chemical species supplying it.…”

Section: Resultsmentioning

confidence: 99%

Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires

et al. 2011

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Photoconductivity is studied in individual ZnO nanowires. Under ultraviolet (UV) illumination, the induced photocurrents are observed to persist both in air and in vacuum. Their dependence on UV intensity in air is explained by means of photoinduced surface depletion depth decrease caused by oxygen desorption induced by photogenerated holes. The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase. After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained. Once vacuum is broken and air is let in, the photocurrent quickly decays down to the typical air-photoresponse values. The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity. The adsorption-desorption balance is fully recovered after the ZnO surface is exposed to air, which suggests that under UV illumination, the ZnO surface is actively "breathing" oxygen, a process that is further enhanced in nanowires by their high surface to volume ratio.

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Section: Resultsmentioning

confidence: 99%

Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires

et al. 2011

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“…15 It is well established that only oxygen, and not CO 2 , can cause the photoconductivity to disappear. [16][17][18] However, the reverse process has been a subject of controversy, with several reports claiming emission of CO 2 alone, 19,20 and one other report claiming emission of O 2 . 21 In spite of the evidence for CO 2 emission, the fact that the reverse process can only take place with O 2 , made many scientists adopt the belief that under "clean" conditions, O 2 should be the only species emitted, and that observations of CO 2 are likely a coincidental result of a non-pure or non-clean material, emission from a metal sample holder, etc.…”

Section: Introductionmentioning

confidence: 99%

Interaction of light with the ZnO surface: Photon induced oxygen “breathing,” oxygen vacancies, persistent photoconductivity, and persistent photovoltage

Gurwitz¹,

Cohen²,

Shalish³

2014

Journal of Applied Physics

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ZnO surfaces adsorb oxygen in the dark and emit CO 2 when exposed to white light, reminiscent of the lungs of living creatures. We find that this exchange of oxygen with the ambient affects the integrity of the ZnO surface. Thus, it forms a basis for several interesting surface phenomena in ZnO, such as photoconductivity, photovoltage, and gas sensing, and has a role in ZnO electrical conduction. Using x-ray photoelectron spectroscopy on ZnO nanowires, we observed a decomposition of ZnO under white light and formation of oxygen-depleted surface, which explains photoconductivity by the electron donation of oxygen vacancies. Our findings suggest that the observed decomposition of the ZnO lattice may only take place due to photon-induced reduction of ZnO by carbon containing molecules (or carbo-photonic reduction), possibly from the ambient gas, accounting in a consistent way for both the reduced demands on the energy required for decomposition and for the observed emission of lattice oxygen in the form of CO 2. The formation of oxygen-vacancy rich surface is suggested to induce surface delta doping, causing accumulation of electrons at the surface, which accounts for both the increase in conductivity and the flattening of the energy bands. Using surface photovoltage spectroscopy in ultra high vacuum, we monitored changes in the deep level spectrum. We observe a wide optical transition from a deep acceptor to the conduction band, which energy position coincides with the position of the so called "green luminescence" in ZnO. This green transition disappears with the formation of surface oxygen vacancies. Since the oxygen vacancies are donors, while the green transition involves surface acceptors, the results suggest that the initial emission of oxygen originates at the defect sites of the latter, thereby eliminating each other. This suggests that the green transition originates at surface Zn vacancy acceptors. Removing an oxygen atom from a Zn vacancy completes the vacancy to become a full ZnO molecule vacancy, which does not produce deep levels. Our results explain why ZnO finds use as an electrical detector for oxygen and for carbon containing gas molecules. They may also shed new light on photocatalytic uses of ZnO. It is suggested that similar surface phenomena may affect other semiconducting oxides. V

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“…Most of the studies of heterogeneous photocatalysis were made using visible or near-u.v. radiation (EA-5 eV) [1][2][3].…”

Section: Light Absorptionmentioning

confidence: 99%